The hydroformylation of 3-pentenoic esters for the preparation of linear 5-formyvalerate esters is a valuable process since 5-formylvalerate esters are important intermediates for the preparation of caprolactam. Processes for preparing 5-formyvalerate esters from hydroformylation of pentenoic esters are disclosed in EPO patent No. 295554 and U.S. Pat. No. 3,253,018. 4-pentenoic ester or acid is used as a reactant to undergo the carbonylation reaction, providing 5-formylvalerate or 5-formyl valeric acid as the major product in both the processes described above. These processes have very limited commercial applications, since 4-pentenoic ester is not readily available from simple reaction procedure. Processes for direct hydroformylation of 3-pentenoic esters are initially disclosed in EPO Patent No. 556,681 and U.S. Pat. No. 5,264,616. The inventions relate to processes for hydroformylation of 3-pentenoic esters with carbon monoxide and hydrogen in the presence of catalyst compositions comprising rhodium carbonyl complexes, and bidentate phosphite ligands of the formula (I) or formula (II): ##STR1##
According to the invention disclosed in EPO Patent No. 556681, the catalyst system induces the hydroformylation of 3-pentenoic methyl ester to yield 5-formyl valerate ester with a reacting selectivity of 76.7 mol %. The disadvantages of these processes are that both phosphite ligand (I) and (II) are prepared from 2,2'-dihydroxy-3,3'-di-tertbutyl-5,5'-dimethoxybiphenyl which is a commercially unavailable starting material. Because of that both ligands derive from expensive raw materials, and thus the cost of preparing these ligands is high.
Other catalyst systems not using 2,2'-dihydroxy-3,3'-di-tertbutyl-5,5'-dimethoxybiphenyl for preparing phosphite ligands are disclosed in WO 95/18089. The catalyst compositions can be used for direct hydroformylation of 3-pentenoic esters in the presence of carbon monoxide and hydrogen to give the desired 5-formyl valeric esters. Again, the catalyst compositions comprise rhodium complexes and a chelating multidentate phosphite ligand of the formula (III): ##STR2##
According to WO/18089, the highest selectivity for the hydroformylation of 3-pentenoic methyl ester to give methyl 5-formyl valerate can reach 80%. The disadvantages of the process are that the chelating phosphite ligand (III) is prepared from the insoluble pentaerythritol, thereby the preparation of the phosphite ligand (III) is difficult and the production cost is high.
All of the catalyst compositions described above for direct preparing 5-formyl valerate from pentenoic esters contain rhodium complexes and chelating phosphite ligands to provide activation ability. It should be noted that during the hydroformylation reaction, the phosphite ligands are easily oxidized and decomposed by hydroperoxide which is found in the presence of the starting pentenoic esters. As described by U.S. Pat. No. 5,527,950, extra amount of the phosphite ligand is required to constantly feed into the catalyst system for maintaining efficient reactivity. Therefore, from economic considerations, it is highly desirable to develop a catalyst composition which contains a low cost phosphite ligand while providing efficient activity toward hydroformylation of 3-pentenoic esters.